Digital data processing systems typically include data storage devices, for example, multi-disk disk drives. Data is recorded on the disks in concentric tracks. The tracks are divided into sectors, and each sector is further divided into a header section and a data section. Data is recorded in (written) or retrieved from (read) the data portions of the sectors while address information, that is, disk surface number, track number and sector number, is stored in the header sections.
Data is read from or written to the disks using read/write heads. Each head is associated with a particular disk surface or portion of a disk surface. As the disks rotate under the heads, data is, for example, read from a particular sector when the associated head is over the data portion of that sector.
To begin a read or write operation the appropriate head is moved to the track containing the desired sector. Unless the head was used in the preceding read or write operation, or it has been kept in synchronization by appropriate timers, for example during idle times, the head must then be synchronized to the disk so that the sector addresses rotating under it can be read. Once the head is synchronized, it locates the selected sector by reading the sector addresses and then it performs the read or write operation. Head switching, which may involve switching between heads on a common actuator arm or switching between heads on different arms, occurs quite often during the normal operation of the disk drive and thus synchronization is performed often also. The speed with which the heads are accurately synchronized to the disks affects the speed with which data can be transferred to or from the drive.
Using prior technology, timing information for synchronizing is recorded either on an extra servo surface or as an additional information zone between sectors, referred to as an embedded-servo zone. If the disk drive is one which uses fixed block architecture and data surface only embedded-servo information, certain information must be contained within the space of each sector and related servo zone to enable the system to operate in real time, that is, to perform certain functions within the time it takes for the sector to rotate under the head. For example, sector timing must be established, and bit, data symbol and data word synchronization must be determined using the recorded servo and synchronization information. Also, if alternate sectors can be used to replace corrupted, or bad, sectors flags indicating which sectors are good and which are bad must be read and interpreted. Additionally, each sector contains data that must be read and validated, and if necessary corrected, using recorded data redundancy symbols. It is desirable to perform all of these functions with minimum disk overhead, that is, with minimum consumption of extra recording space, leaving the disk space free for data.
Prior technology embedded-servo disk drives typically use three-zone formats. The three zones are a servo zone, containing servo position information which is permanently written at the factory when the disks are formatted, and two sector zones, a header zone containing address and bad sector information which can be re-written during re-formatting to indicate which sectors have degraded from good to bad, and a data zone containing data and data redundancy symbols which can be easily written and re-written by the user.
Using three zones requires the disk drive to synchronize three times, once per zone, in order to perform a read or write operation in a sector. Thus each zone must contain synchronization and timing information. This causes increased overhead, resulting in a loss of recording efficiency. Thus it is desirable to reduce the number of zones while retaining all the functionality of the three-zone architecture. Also, as users demand disk drives with more and more data storage capacity, it is more and more desirable to use less disk space for synchronization.
Prior technology also requires significant dedicated timing hardware, for example, timing logic or analog electronics is required to interpret the recorded timing information and synchronize the heads. The servo-zone typically contains a long synchronization burst written in a frequency which is unlikely to occur in the data, servo information for track following, and several information bits for track identification. Thus the embedded servo system must use analog filters to locate the synchronization burst and special timing and control logic to find the servo information, as well as to synchronize the head to the servo zone. This additional hardware increases the cost of the system. Thus it is desirable to perform synchronization using much of the same hardware as the normal read and write operations, and in particular using mostly digital logic. It is also desirable to perform synchronization quickly, even if the heads are not in a known position with respect to the servo information that would be used to align the data head on the track.